A typical optical network is made up of optical filter elements such as multiplexers, demultiplexers, interleavers, deinterleavers, wavelength selective switches or any other element that has filtering characteristics. As an optical signal passes through the optical network, it crosses various filter elements. When crossing an optical filter element, the optical signal is likely to deteriorate as it is processed. For example, the bandwidth of the optical signal may change due to the filter element through which it passes, or the optical power of the signal may change due to passing through a filter element.
The transfer function of a filter element provides a measure of the amount of deterioration of an optical signal as it passes through that filter element. For example as a signal passes through a limited bandwidth channel of a filter element, the signal may lose part of its optical power due to the limited bandwidth of the channel. In order to account for such power loss, manufacturers of the optical filters provide specifications which are relied upon by system designers when designing the optical network.